A scramjet engine is a supersonic combustion ramjet, which unlike a conventional ramjet wherein inlet airflow and combustion gases flow subsonically, the inlet airflow and combustion gases of a scramjet flow supersonically. Conventional ramjets operate over a range of aircraft speeds from about Mach 3 to about Mach 6, and, conventional scramjets operate from about Mach 5 and above. The relatively high speed of an aircraft utilizing a scramjet in the flight regime of about Mach 5 or above is also referred to as hypersonic velocities.
It is not believed that a scramjet-powered aircraft has yet been built and flown. However, small research-type scramjet engines have been built and laboratory tested at simulated flight speeds up to about Mach 7. Accordingly, the references herein to conventional and typical scramjets and structures refers to information conventionally known to those skilled in the art of engines for powering aircraft at supersonic velocity, which is based, in part, on mathematical modeling and analysis.
Since scramjets operate on supersonic fluid flow therethrough, they are fundamentally different in structure and operation from conventional ramjet engines which operate with subsonic fluid flow therethrough. A typical scramjet engine includes a supersonic inlet, or diffuser, for compressing inlet airflow followed by a supersonic combustor and in turn followed by a supersonic exhaust nozzle. A supersonic inlet is a converging nozzle leading to a throat, and the supersonic nozzle is a diverging channel which is in flow communication with the throat. The combustor typically extends from the throat and is a constant area or diverging channel formed integrally with the diverging exhaust nozzle.
Fuel is added to the supersonic inlet airflow in the combustor for combustion which usually is spontaneous combustion since the inlet airflow has been compressed to temperatures of about 2000.degree. R. and higher. However, since the fuel, which is typically gaseous hydrogen having relatively low momentum is injected into a supersonic airstream which has relatively high momentum, effective penetration of the fuel into the air and across the combustor and effective mixing of the fuel with the air is difficult to obtain, especially at high hypersonic flight speeds.
To permit the required internal compression at supersonic and hypersonic inlet airflow Mach numbers, means are conventionally provided to allow the inlet to "swallow" the normal shock which would otherwise stand at the entrance to the inlet. A conventional ramjet engine experiences such normal shock which decelerates the inlet airflow to subsonic Mach numbers with associated large losses in efficiency, relatively high heating rates, large structural loads, and relatively large drag over the ramjet engine. The inlet of a scramjet engine is considered "started" once the normal shock has been "swallowed" by the scramjet engine which results in supersonic airflow in the scramjet.
In the event of a sudden increase in back pressure occurring in the scramjet engine, such as for example, from high fuel flow transient, sudden angle of attack change of the aircraft, or local choking of the internal airflow, then the inlet may "unstart" with the attendant undesirable characteristics described above. This can be a significant problem for an aircraft operating at hypersonic velocity since that aircraft was designed with a scramjet engine having internal supersonic airflow. If the scramjet engine unstarts, the flow of air therethrough is no longer supersonic and since the aircraft is operating at hypersonic velocity, it is impossible to restart without additional means. Such means typically include conventionally known variable internal inlet geometry and high response control systems to permit inlet restarting. Since the inlet is typically initially started at relatively low hypersonic Mach numbers, any subsequent unstarts at relatively high hypersonic Mach numbers can represent a significantly more severe condition which must be accommodated.
Since a scramjet powered aircraft operates at substantial hypersonic velocity, drag over the scramjet engine is a substantial factor which should be minimized. However, variable internal inlet geometry scramjets are inherently relatively complex and relatively large and provide increased surface area with attendant relatively large drag associated therewith. A substantial weight penalty is also imposed.
Furthermore, a scramjet engine is typically utilized in an aircraft wherein engine inlet airflow is initially compressed externally of the scramjet by oblique shock waves from the aircraft bow, and the exhaust from the scramjet engine is typically channeled generally parallel to an inclined afterbody of the aircraft for providing external expansion of the supersonic combustion gases from the scramjet engine. Accordingly, the scramjet engine inlet airflow is typically channeled along the bow at an acute angle to the longitudinal or axial axis of the scramjet engine and then must be turned back away from the longitudinal axis to flow generally parallel to the inclined aircraft afterbody. This turning of hypersonic fluid flow first toward the longitudinal axis and then away from the longitudinal axis requires a suitable length of the scramjet engine which typically is relatively large therefore increasing the scramjet engine external surface area and drag. This turning of the scramjet inlet airflow back to the longitudinal or axial axis typically requires the introduction of flow expansion of the turning hypersonic fluid flow which inherently reduces the static temperature and static pressures from the levels achieved by the external compression, thusly substantially wasting much of the external compression which was paid for with irretrievable inlet losses and heating.